1. Field of the Invention
The present invention relates to cationic electrodeposition compositions and, more particularly, to cationic electrodeposition compositions for deposition over rough steel.
2. Brief Description of the Prior Art
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection and low environmental contamination. Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
One disadvantage associated with conventional cationic electrodeposition compositions is that they do not form smooth cured coatings over rough steel substrates. They usually deposit as thin films, that is, from about 10 to 20 microns, which, after curing, do not mask the surface roughness of the underlying steel.
Recently, cationic electrodeposition compositions have been developed which deposit as relatively thick films, that is, from about 25 to 50 microns. Typical of these compositions are those which contain blocked isocyanate curing agents and which are available from PPG Industries, Inc. under the trademark UNI-PRIME. These coating compositions have been enthusiastically received by the automotive industry because they can be deposited on smooth steel substrates and topcoated without the need for primer surfacer. The resultant composite coatings have excellent appearance and chip resistance over smooth steel.
However, these coating compositions are also not completely satisfactory for primer use over rough steel substrates which are often used in automobile manufacturing. Although providing for good chip resistance, the resultant cured coating is surprisingly rough and this surface roughness causes the topcoat to reflect images in a blurry fashion which is not acceptable for automotive applications. An application of an intermediate coating (primer surfacer often sanded) is required to provide a smooth enough surface for passenger cars with acceptable topcoat appearance.
The cause of this surface roughness was at first not understood because when the thicker film build compositions were initially deposited, they completely filled the surface irregularities of the rough steel substrate. The uncured coatings appeared very smooth. However, upon curing, the coating became very rough.
Although not intending to be bound by any theory, it is believed that the primer coat which contains blocked isocyanate curing agents shrinks during cure. A principal cause of the shrinking was believed to be due to the loss of blocking agent, i.e., relatively high molecular weight alcohols such as 2-ethylhexanol and 2-hexoxyethanol. It was found that after dehydration and removal of organic solvent, the uncured film would lose about 15-20 percent of its weight upon final curing. Since the film was crosslinking, no or very little dimensional relaxation occurred to compensate for the weight loss and a high volume shrinkage resulted.